1. Field of the Invention
The present invention relates to flowable liquid suspensions of phosphor powders and methods for the deposition of phosphor powders from such liquid suspensions using direct-write tools such as an ink-jet or similar device. The present invention is also directed to articles of manufacture, such as displays and security indicia, formed by direct-write deposition of phosphor powders.
2. Description of Related Art
Phosphors are compounds that are capable of emitting useful quantities of radiation in the visible and/or ultraviolet spectrums upon excitation of the material by an external energy source. Due to this property, phosphor compounds have long been utilized in cathode ray tube (CRT) screens for televisions and similar devices. Typically, phosphor compounds include a host material doped with a small amount of an activator ion. A wide variety of phosphor compounds are known and many different colors can be provided.
Phosphor powders are also utilized in many advanced display devices that provide illuminated text, graphics or video output, including flat panel display devices such as liquid crystal displays, plasma displays, thick film and thin film electroluminescent displays and field emission displays.
Liquid crystal displays (LCD's) use a low powder electric field to modify a light path and are commonly used in wristwatches, pocket televisions, gas pumps, pagers and the like. Plasma displays utilize a gas, trapped between transparent layers, that emits ultraviolet light when excited by an electric field. The ultraviolet light stimulates phosphors on the screen to emit visible light. Plasma displays are particularly useful for larger displays, such as greater than about 20 diagonal inches. Thin film and thick film electroluminescent displays (TFEL's) utilize a film of phosphorescent material, trapped between glass plates and electrodes, that emits light under an applied electric field. Such displays are typically used in commercial transportation vehicles, factory floors and emergency rooms. Field emission displays (FED's) are similar in principle to CRT's, in which electrons emitted from a tip excite phosphors, which then emit light of different color. Phosphor powders are also utilized in electroluminescent lamps (EL's), which include a phosphor powder layer deposited on a polymer substrate in which the powder emits light when an electric field is applied.
Many of the foregoing devices are produced using complicated and labor-intensive production steps for the deposition of the phosphor powders. For example, many of these devices are formed by depositing the phosphor powder using a thick-film paste method. In this method, a past containing the phosphor particles is deposited by squeezing the paste through a porous screen having a desired pattern of apertures. Other methods include depositing the phosphor powder electrophoretically or by selectively coating the display screen with a slurry. Presently, there are no faster or more adaptable processes for depositing the phosphor powder. Further, there are no known processes that permit the rapid prototyping of new devices, particularly since the formation of a screen for applying a thick film paste or the patterning steps for slurry deposition are costly and labor intensive.
In addition to display applications, phosphors can also be used to create security indicia for deterring fraud and counterfeiting. In this application, phosphors which are not visible under normal lighting conditions can be illuminated by ultraviolet light, or the like, to positively identify an article by the indicia pattern and/or the wavelength of the emitted light.
For example, U.S. Pat. No. 5,569,317 by Sarada et al. discloses fluorescent and phosphorescent tagged inks for indicia, such as postal indicia used to stamp envelopes. The inks can include fluorescent and phosphorescent organic dyes and can also include phosphorescent compounds such as rare earth metal sulfides. It is disclosed that the inks can be applied using a variety of known methods.
U.S. Pat. No. 5,792,380 by Wen et al. discloses an ink-jet printing composition including a detectable label material. The detectable label material is a magnetic material, such as a ferric oxide.
The deposition of phosphor powders using a direct-write tool such as an ink-jet has not between successfully carried out, primarily due to the need for phosphor powder having acceptable physical and luminescent characteristics. It would be advantageous to provide compositions and methods for depositing phosphor particles in a manner such that devices, such as flat panel displays, could be more easily produced or prototyped and so that secured articles can be easily produced using luminescent compositions that are not normally visible but are detectable using ultraviolet light or other means.